As well known, control rods in nuclear reactors perform dual functions of power distribution shaping and reactivity control. The rods enter from the bottom of the reactor and are typically connected to bottom-mounted, hydraulically actuated drive mechanisms which allow either axial positioning for reactivity regulation or rapid scram insertion. The control rod-to control rod drive connection permits each control rod to be attached and detached from its drive during an outage, for example, during refueling, without disturbing the remainder of the control system for the control rod.
The control rods are generally cruciform in cross-sectional shape, with each panel or wing of the rod containing tubes filled with boron carbide capsules or hafnium rods or empty capsules or combinations of the foregoing. In one particular prior control rod, there are provided a plurality of generally square-shaped absorber tubes welded to one another to form the panels. The tubes, of course, are variously loaded with neutron absorber materials and are fixed in relation to the control rod. In a second form of prior control rods, absorber tubes filled with neutron absorber materials are disposed within sheathing defining the panels or wings of the control rod. The sheathing is welded to the tie rod and to the handle and transition piece at respective opposite ends of the control rod. The absorber tubes are similarly fixed against movement and absorb compressive stresses. In each prior control rod described above, the absorber tubes are connected one to the other and thus are not readily individually replaceable nor is it generally possible to handle the tubes to provide different loadings of neutron absorber material into the various absorber tubes.
In the last-mentioned prior art control rod, the absorber tubes within the sheaths contained B.sub.4 C capsules in direct contact with the interior surfaces of the tubes. Thus, after insertion into the nuclear reactor and immediately upon irradiation, the capsules give off helium, which swells the tube and applies a stress and strain to the tube walls, resulting in stress corrosion cracking of the tube. In the first-mentioned prior art control rod described above, the B.sub.4 C capsules are inserted into the generally square absorber tubes with clearances between the interior cylindrical surfaces of the tubes and the outer surfaces of the capsules such that, at 40% burn-up, the capsules will engage the walls and initiate stresses and strains within the tubes. While this minimizes stress corrosion cracking, it is only a deferral of the internal pressure build-up to a predetermined time within the life-cycle of the control rod. It is not a solution to the problem of stress within the absorber tubes. That is, there is a mechanical limit for both the foregoing described control rod designs based on the internal pressure and the induced strain in the tube, before the internal pressurization causes the tubes to possibly rupture. Moreover, it will be appreciated that in both of the foregoing described designs, the individual tubes are welded together, thus essentially precluding the possibility of replacement, rearranging or inverting individual absorber tubes or replacing entire absorber tube sections of the various panels. The sheath design also prevents inspection of the absorber tubes while in the reactor.